For women in many parts of the world, the behavior that puts them at greatest risk for HIV infection is unprotected sex within marriage. This comparative ethnographic study explores how social and cultural factors influence marital and extramarital intimate relationships and examines the sexual and the HIV prevention practices of men and women engaged in building these relationships across five locations in countries at different stages of the HIV epidemic. By detailing the processes through which contextual factors shape women's risk of marital HIV infection, this study contributes to our understanding of ways to reduce the risk of heterosexual HIV transmission.